An image intensifier (I2) tube amplifies light to provide a visible image of a scene. Typically, the I2 tube includes a photocathode (PC) behind the light-receiving face of the tube. The PC is responsive to photons of visible and infrared light to liberate photoelectrons. Because an image of a scene is focused on the PC, photoelectrons are liberated from the PC in a pattern which replicates the scene. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a multitude of microchannels. These microchannels have an interior surface at least in part defined by a material liberating secondary-emission electrons, when photoelectrons collide with the interior surfaces of the microchannels. In other words, each time an electron (whether a photoelectron or a secondary-emission electron previously emitted by the microchannel plate) collides with this material at the interior surfaces of the microchannels, more than one electron (i.e., secondary-emission electrons) leave the site of the collision.
As a consequence, the photoelectrons entering the microchannels cause a geometric cascade of secondary-emission electrons moving along the microchannels, from one face of the microchannel plate to the other face, so that a spatial output pattern of electrons issues from the microchannel plate. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the microchannel plate impacts on and is absorbed by the phosphorescent screen electrode, visible-light phosphorescence occurs in a pattern which replicates the image. This visible-light image is passed out of the tube for viewing via a transparent image-output window.
It is estimated that about 20% of the electrons from the photocathode that impinge on the input surface of the MCP are scattered back toward the photocathode. The backscattered electrons are repelled by the electric field between the photocathode and the input surface of the MCP and forced to strike the input surface of the MCP a second time. This causes what is known as a halo effect, resulting in the electrons spreading out from the size of a small spot at the photocathode to the size of a much larger spot at the input surface of the MCP. A similar backscattered electron halo-generating effect also takes place at the phosphorescent screen.
In order to suppress this effect at the phosphorescent screen, a collimator is included in some image intensifier tubes. Such a collimator is disclosed in U.S. Pat. No. 5,495,141 and incorporated herein by reference. As described therein, a collimator is inserted between the output surface of the MCP and the phosphor screen. Some of the electrons entering the collimator strike the collimator walls and are prevented from reaching the phosphor screen. This phenomenon, however, reduces the number of electrons that get through the collimator to about 25% to 50% of the electrons leaving at the output of the MCP. This, in turn, results in a brightness loss for the image intensifier tube.
As will be explained, there are other problems resulting from attempts to reduce halo effects in image intensifier tubes. The present invention advantageously overcomes some of these problems and produces an image intensifier tube with reduced secondary emissions, reduced halo in the output image and reduced charge build-up that causes image burn-in and may damage the image intensifier tube.